This invention relates to a scintillation detector for nuclear radiation.
When certain materials are subjected to nuclear radiation, atoms or molecules in the material are raised to a higher energy state. When the atoms or molecules revert to their ground state, light may be emitted. If light is emitted only during excitation, the phenomenon is called fluorescence, while if substantial light emission occurs after the incident radiation has ceased, the phenomenon is called phosphorescence (after-glow).
When nuclear radiation is incident on the material, ionization occurs, creating a large number of charged carriers. The number of electron-hole pairs created is proportional to the energy of the incident radiation. If suitable centres, known as luminescence centres, are present, the electron-hole pairs re-combine at the centres, giving off light. The intensity of the light emitted by the material is proportional to the energy of the incident radiation. The total number of light pulses produced gives an indication of the intensity of the incident radiation, and the distribution of the magnitude of the pulses is related to the incident particle energy. The detection of radiation by means of the above described phenomenon is known as scintillation counting.
The possibility of using natural diamond as a scintillation counter has been recognised for some time. However, most natural diamonds and also synthetic diamonds exhibit a long after-glow. In the case of synthetic diamond, an after-glow of up to several hours is possible. This results in poor time and energy resolution, and results from the presence of trapping centres in these diamonds. The trapping centres remove a certain fraction of the charged carriers of either sign produced by the incident radiation, resulting in energy degradation and poor time resolution. A subsequent vacating of these traps gives rise to varying degrees of phosphorescence. The presence of this phosphorescence "tail" makes such diamonds unsuitable for use as scintillation counters, since the count rates of these diamonds are not directly proportional to radiation intensity, and they may display relatively high background count rates even after cessation of the incident radiation.